[Greenbuilding] Siding over foam insulation?

[Alan Abrams]

<One of the bidders for our siding job on our house, in response to my 
question about installing an inch or two of foam board over my 
sub-sheeting on my walls, said that the warranty for the siding would be 
voided if installed in that manner.  (It is only 2x4 bays currently, 
hence the desire for thicker insulation.  And the foam sheets sounds 
easier/better than tacking on a 2" furring strip over each of the studs 
to increase the bay depth to 6".  Which in turn sounds easier than doing 
an offset stud, or something along those lines, to increase the bay depth.)

So my question is, what about installing the foam under the 
sub-sheeting?  It would go like this: remove all the current sheeting to 
expose the stud bays, insulate the walls, install foam sheets, install 
wall sheeting (and probably a wrap), install siding.  I would use long 
enough screws on the sheeting to reach through the foam to the studs, of 
course.  But then the siding would be going into the sheeting only.>


this concept is feasible; in a way it is as sensible as wearing a sweater
under a rain coat on a cold wet day.  

the caveat is resistance to wind load.  the sheathing, nailed tight to the
studs, creates shear panels that resist the racking force of wind.  even an
inch of separation would compromise this function.

a solution to this problem would be to use diagonal let-in wood braces, or
metal rack braces.  There are a host of variables to consider here,
including design wind speed for your region, dimensions of each wall, and
configuration and location of openings.  The geometry of the roof is a
factor as well.

i wouldn't let this be discouraging--all this is not as complicated as it
sounds; it's everyday stuff to an experienced engineer.  

Alan Abrams, AIBD
Abrams Design Build
a sustainable approach to beautiful space
alan at abramsdesignbuild.com
www.abramsdesignbuild.com
202-726-5894 o
202-291-0626 f